1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a thin film transistor array substrate and a fabricating method thereof that are adaptive for improving an aperture ratio.
2. Discussion of the Related Art
Generally, liquid crystal displays (LCD) control light transmittance of liquid crystal using an electric field to thereby display pictures. Liquid crystal displays are largely classified into a vertical electric-field-type and a horizontal electric-field-type depending upon a direction of the electric field driving the liquid crystal.
An LCD of the vertical electric-field-type drives a liquid crystal in a twisted nematic (TN) mode with a vertical electric field formed between a pixel electrode and a common electrode arranged in opposition to each other on the upper and lower substrates. The LCD of the vertical electric-field-type has an advantage of a high aperture ratio while having a drawback of a narrow viewing angle of about 90°.
An LCD of the horizontal electric-field-type drives a liquid crystal in an in plane switch (IPS) mode with a horizontal electric field between a pixel electrode and a common electrode arranged in parallel with each other on the lower substrate. The LCD of the horizontal electric-field-type has an advantage of an wide viewing angle of about 160°.
Hereinafter, a liquid crystal display of the horizontal electric-field-type will be described in detail.
FIG. 1 is a plan view illustrating a structure of a thin film transistor array substrate of a horizontal electric-field-type LCD according to the related art, and FIG. 2 is a cross-sectional view of the thin film transistor array substrate taken along a II-II′ line in FIG. 1.
Referring to FIGS. 1 and 2, a thin film transistor array substrate according to the related art includes a gate line 2 and a data line 4 on a lower substrate 1 in such a manner to cross each other. The thin film transistor array substrate further includes a thin film transistor 30 near each crossing, a pixel electrode 22 and a common electrode 24 at a pixel area defined by the crossing between the gate and data lines for forming a horizontal electric field. The common electrode 24 is connected to a common line 26.
The gate line 2 applies a gate signal to a gate electrode 6 of the thin film transistor 30. The data line 4 applies a pixel signal, via a drain electrode 10 of the thin film transistor 30, to the pixel electrode 22. The common line 26 is formed in parallel to the gate line to apply a reference voltage to the common electrode 24 for driving the liquid crystal.
The thin film transistor 30 allows a pixel signal of the data line 4 to be charged and maintained in the pixel electrode 22 in response to a gate signal of the gate line 2. To this end, the thin film transistor 30 includes the gate electrode 6 connected to the gate line 2, a source electrode 8 connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22. Further, the thin film transistor 30 includes an active layer 14 overlapping the gate electrode 6, with a gate insulating layer 12 therebetween to define a channel between the source electrode 8 and the drain electrode 10, and an ohmic contact layer 16 for making an ohmic contact with the source electrode 8 and the drain electrode 10.
The pixel electrode 22 is connected, via a contact hole 20 in a protective layer 18, to the drain electrode 10 of the thin film transistor 30 and is provided at the pixel area 5. In particular, the pixel electrode 22 includes a horizontal part 22a connected to the drain electrode 10 and provided in parallel to adjacent gate lines 2, and a finger part 22b protruded from the horizontal part 22a in a direction parallel to the common electrode 24.
The common electrode 24 is connected to the common line 26 and is provided at the pixel area 5. In particular, the common electrode 24 is provided in parallel with the finger part 22b of the pixel electrode 22 at the pixel area 5.
Accordingly, a horizontal electric field can be formed between the pixel electrode 22 to which a pixel signal is supplied via the thin film transistor 30 and the common electrode 24 to which a reference voltage is supplied via the common line 26. In particular, a horizontal electric field can be formed between the finger part 22b of the pixel electrode 22 and the common electrode 24. When such a horizontal electric field is applied between the pixel and common electrodes 22 and 24, the liquid crystal molecules arranged in a horizontal direction between the thin film transistor array substrate and a color filter array substrate (upper substrate) rotate due to a dielectric anisotropy of the liquid crystal. An amount of light transmittance at the pixel area varies depending upon an extent of rotation of the liquid crystal molecules, thereby implementing gray-scale levels.
In the liquid crystal display of the horizontal electric-field-type according to the related art, the liquid crystal molecules at an area “A” between the common line 26 and an edge of the finger part 22b of the pixel electrode 22 and at an area “B” between an edge of the common electrode 24 and the horizontal part 22a of the pixel electrode 22 tend to respond differently than the liquid crystal molecules at other areas (for example, top part), when it is supplied with an electric field. In other words, the liquid crystal molecules at the areas “A” and “B” degrade picture quality by, for example, creating disclination, because they have a rotation angle and a rotation direction different from those of the liquid crystal molecules at other areas. As a result, there is a limit in increasing the aperture ratio to enhance the light transmittance of the LCD.